In recent years there has been an increasing interest in the manufacture of composite materials, i.e., materials that are made up of more than one constituent mixed into another constituent. Such composite materials have significantly improved the performance of ceramics, metals and plastics. Ceramic composites, for example, have utilized silicon carbide whiskers to reinforce aluminum oxide in order to make cutting tools which significantly outperform competitive materials. With hardness values of 20-22 GPa and toughness values in the range of 6-8 MPa.m.sup.1/2, the Al.sub.2 O.sub.3 -SiC cutting tools significantly outperform competitive materials, such as tungsten carbide. These advanced composites are claimed to machine nickel-based alloys ten times faster, and last up to twenty-five times as long based on the amount of metal cut.
Silicon carbide whiskers, when added to aluminum, have been shown to provide an improvement in strength up to a factor of three over that exhibited by aluminum alone. Modulus of elasticity improvements of two to two and one half times have been demonstrated. Similar results have been obtained with composites of other metals.
Composite materials are typically manufactured by adding a reinforcement material, such as the silicon carbide, to a baseline starting material referred to as the matrix material. The matrix may be either a metal, a ceramic or a polymeric material. In addition to the silicon carbide reinforcement material, other ceramic materials (or materials that exhibit a ceramic-like behavior) are often used. These tend to have high strength, high modulus of elasticity and low densities. The specific combinations are chosen to provide improvements in strength, stiffness (modulus of elasticity), wear resistance, creep behavior, hardness and higher strength-to-weight ratios. Other properties that can be affected by the choice of the composition are the adjustment of thermal coefficient of expansion, thermal conductivities and total weight.
Although a number of reinforcement materials have been utilized, the most commonly used materials are carbon fibers and silicon carbide whiskers and fibers. A whisker is defined as a single crystal material, and a fiber as a polycrystalline material. Because SiC whiskers do not have any chemical compatibility problems such as those exhibited by carbon fibers in metals and ceramics, these have emerged as the normal reinforcement material of choice for most metal and ceramic composite work.
Some of the prior work in this field of SiC reinforcement is described in U.S. Pat. No. 4,543,345 issued to G. C. Wei that discusses a composition for ceramics with SiC whiskers in alumina, mullite or boron carbide. Another is U.S., Pat. No. 4,652,414 issued to T. Tiegs which describes a refinement of the earlier patent that results in the manufacture of complex shapes; however, the process is rather complex. Still another piece of work in this field is that described in U.S. Pat. No. 4,657,877 issued to P. Becker and T. Tiegs. This describes the use of zirconia and SiC whiskers to produce a product that can be operated at a higher temperature than when only SiC is used.
The use of SiC whiskers is not without problems. One problem encountered by the additions of SiC whiskers is that many of the manufacturing processes produce a submicrometer diameter whisker which is very similar to asbestos in size and shape. Consequently, the manufacturing steps require caution at each step until the whiskers are locked solidly within a composite matrix.
In order to provide the desired benefit to the composite, the SiC whiskers should be smooth and contain no particulates. However, in actual practice, the whiskers almost always contain particulates and clumps of whiskers bonded together. Further, the whiskers often have a rough surface which reduces their effectiveness to toughen a ceramic. When there are particulates or clumps of whiskers, the location of such in the product serves as a crack initiation site. The surface condition of the whisker often adversely affects the strengthening characteristic, with carbon and silicon dioxide being two typical impurities often found in commercial SiC whiskers. This is particularly true when such whiskers are utilized to strengthen an aluminum oxide matrix.
Furthermore, the use of silicon carbide whiskers, due to their cost, results in a rather expensive final product.
In the aluminum industry there are a wide variety of aluminum alloys produced that are "tailored" by chemistry, heat treatment and fabrication techniques for particular uses. Some of the compositions include the addition of boron as alloying agents. Typical of these alloys is that described in U.S. Pat. No. 4,595,559 issued to C. Planchamp on June 17, 1988. This patent discusses the addition of powders of AlB.sub.2 or AlB.sub.12 to molten aluminum to achieve up to 30% boron in the alloy. Another reference in the aluminum-boron art is U.S. Pat. No. 3,503,738 issued to H. S. Booper on Mar. 31, 1970. Potassium borofluoride is reacted with aluminum, and the released boron is alloyed with excess aluminum. This forms a "master alloy" that is used to add boron to aluminum alloys as a grain refiner generally in the presence of titanium.
Other references in this field are: U.S. Pat. No 4,6311,236 issued to H. Rocyzn on Dec. 23, 1986, in which boron-containing powders (e.g. boron carbide) are added to manufacture shapes by powder metallurgy processes, plus extrusion, to produce a core for inclusion within aluminum or aluminum alloys for use as absorbers for nuclear reactors; U.S. Pat. No. 4,604,368 issued to M. Reeve that discusses reacting potassium borofluoride with molten aluminum, and then the boride powder is removed from molten metal by filtering, with the powder subsequently reacted with titanium nitride or titanium hydride to form titanium boride; U.S. Pat. No. 4,647,405 issued to P. E. Debely on Mar. 3, 1987 describes the use of titanium oxide, B.sub.2 O.sub.3 and aluminum powders to form an alumina-titanium boride composite; and U.S. Pat. No. 4,755,221 issued to M. Paliwal, et. al, on July 5, 1988 describes the use of a plasma to form a composite of titanium boride with aluminum metal.
None of the above-cited references result in a process wherein the composition is a composite that is easily tailored for particular applications where the combinations of hardness and toughness are required. Further, some of the processes are quite hazardous and others are very expensive.
Accordingly, it is an object of the present invention to provide a process for the manufacture of composites containing aluminum oxide, aluminum boride and aluminum.
It is another object of the present invention to provide a process for the strengthening and toughening of aluminum and aluminum alloys that is inexpensive and has a lower health risk than the whisker inclusion technique of the prior art.
It is a further object to provide a process for producing tough ceramic products containing aluminum oxide, aluminum boride and aluminum metal.
An additional object of the present invention is to provide a process for the production of products containing aluminum oxide, aluminum boride and aluminum metal that has a very high yield of product.
Also, it is an object to provide a ceramic composite having at least aluminum oxide and aluminum boride, and typically containing aluminum metal, whose hardness and toughness properties can be easily tailored for a particular application.
These and other objects of the present invention will become apparent upon a consideration of the detailed description of the invention that follows.